This invention relates to antennas and more particularly to a wide bandwidth antenna array manufacturable in a slimline flat pack configuration.
Dish type microwave antennas have for some time been located on ships where they are vulnerable to attack as well as damage in ocean going conditions. As a result, there has been an effort to provide so-called xe2x80x9csmart skinxe2x80x9d antennas in which the antennas are configured in a flat panel array and become part of the structure in which they are embedded, namely for instance a wall of a deck house.
In order to provide a flat panel antenna a standard slot antenna array is fed with a balanced line feed directly connected to opposed sides of the slot. A 180-degree hybrid is used to convert an unbalanced line such as a coaxial cable to a balanced feed. One of the problems with such a direct coupled balanced line feed for a slot antenna is the relatively narrow bandwidth of the resulting antenna. In the usual instance the percent bandwidth is approximately 10%, such that for an antenna tuned to 100 MHz, the operating frequency range would be 100 MHz plus or minus 5 MHz.
In an effort to improve on the operating frequency range of slot antennas, stripline feeds have been devised in which a conductive strip is mounted transverse to the slat on the underneath side of a dielectric layer on top of which a slotted conductive layer is patterned, with the stripline either terminated in a resistive load or in an approximately xc2xcxcex long radial stub.
While the bandwidth of such a stripline fed slot antenna is indeed better than the standard antenna, its 25% bandwidth still does not provide the type of frequency coverage that one would like. For instance with a 25% bandwidth for a 100 MHz center frequency, the frequency range of the antenna is 100 MHz plus or minus 12.5 MHz.
More importantly, when utilizing stripline to feed slot antennas, it is only with difficulty that one can obtain an array of slot antennas due to the fact that the striplines must, of necessity, cross each other, making feeding of these antennas virtually impossible. Thus, even though there is a theoretic increase in the bandwidth of slot antennas fed with terminated striplines, arrays of these slotted antennas have proved to be elusive from the point of view of manufacture.
In order to solve the problem of providing a single wide bandwidth flat panel cavity-backed antenna array, in the subject invention each of the slots of the array is fed by a dipole radiator which, in one embodiment consists of a pair of tear drop shaped pads underneath an associated slot, with the pads being spaced from the slot by a dielectric layer. In a preferred embodiment, the tear drop shaped pads are positioned to either side of the slot antenna at the mid point of the slot. Off-center feeds are also contemplated and are within the scope of the subject invention. Thus, rather than traversing the slot with a stripline, only a limited area pad need be provided underneath a slot at its mid-point to be able to drive the antenna.
In one embodiment, the dipole elements are fed by an upstanding printed circuit balun such as that described in U.S. Pat. No. 6,452,462 issued to Zane Lo on Sep. 17, 2002 and assigned to the assignee hereof, with an upstanding printed circuit balun underneath each dipole, there need be no crossed striplines. This makes arraying the slot antennas possible. Note that there are other types of baluns usable to feed each dipole, and the subject invention is not limited to the particular balun used.
What makes the dipole driven slot antenna attractive as a wide bandwidth antenna is the fact that the slot antenna is driven by another antenna, namely the dipole radiator, rather than by a capacitative coupling or a balanced line hard connection.
The dipole radiator can be configured such that its impedance characteristics match the impedance characteristics of the slot antenna such that the two impedance characteristics match from the low frequency end of the antenna to the high frequency end. Thus, rather than having an impedance match which only matches at for instance to a 50 ohm for coaxial cable and thus results in a narrowband antenna, utilization of the dipole radiator with its matching impedance results extremely wideband antenna. This being the case, in one embodiment the present bandwidth of the combined dipole radiator fed slot antenna is in the 70% range, meaning that for a 100 MHz center frequency, for example, the frequency range of the antenna is 100 MHz plus or minus 35 MHz.
Additionally, because the dipole feed is of such limited territorial extent, the array may be configured such that a first set of low frequency antenna slots can be interspersed between another array of high frequency slots utilizing the same real estate and the same substrate such that the slots are formed in the same conductive ground plane.
It is therefore possible, for example, to provide an antenna array operating between 100 and 200 MHz, with another array operating between 200 and 400 MHz. Because there are no overlapping of frequencies, there is only negligible cross talk between the two antenna arrays. The result is a 100-400 MHz array in the example mentioned above, with the two arrays being co-planar and co-extensive, although interleaved. Note that 100-400 MHz is just an example. The operating frequency is actually scalable to all other frequency bandwidths. For example, one can devise an array having a frequency range from 500 to 2000 MHz, or 1 GHz to 4 GHz.
Utilizing the printed circuit baluns described in U.S. Pat. No. 6,452,462, it is possible to construct an array of slot antennas in a slim flat pack version, in which the thickness of the flat pack is between one and a half and two inches in the example mentioned above. This means that the array can be embedded into the sidewall of a vessel""s deck house, the side of a turret, or into any upstanding vessel wall. An antenna thus constructed can also be made as part of a building structure such as the wall of a building. Note that in either case are there unsightly and fragile dish antennas.
In summary, a cavity-backed wideband slimline flat panel antenna array for providing a steerable beam or multiple beam includes an array of slot antennas, each of which fed by its own individual dipole radiator, with the wide bandwidth being due to the matching impedances of the slot antenna and dipole radiator across the entire frequency band. In one embodiment, an upstanding printed circuit balun feed is connected to each dipole. The dipole elements are located to either side of a slot and are arrayed on the underneath side of a dielectric layer under the substrate into which the slots are formed, with the dipole elements directly fed by individual upstanding printed circuit baluns, as one of the many balanced feed approaches, which are arrayed beneath the individual slots antennas. The use of the dipole elements, in addition to providing a wider operational bandwidth also permits feeding each of the slots without having to use striplines which would have to cross each other in an array configuration and therefore not possible to work. A wide bandwidth steerable flat panel array utilizing the dipole fed slot antennas may be mounted on the deck house or other flat structural component of a vessel so as to perform a xe2x80x9csmart skinxe2x80x9d function in which the antenna not only functions as a radiating element, but also as a structural part of the vessel itself. In commercial applications, the flat panel array may be incorporated into the wall of a building such that point-to-point communications between buildings may be accomplished through an antenna which is also a structural part of the building. Note that the beams from the antenna are aimable by appropriately phasing the array to point at a receiving antenna on an adjacent building.